1. Field of the Invention
The invention relates to methods for treating subterranean formations. Specifically, the invention is directed to methods of fracturing a subterranean formation utilizing a fracturing composition which includes an encapsulated enzyme breaker.
2. Description of the Prior Art
It is a common practice to treat subterranean formations to increase the gross permeability or conductivity of such formations by procedures which are identified generally as fracturing processes. For example, it is a conventional practice to hydraulically fracture a well in order to produce one or more cracks or "fractures" in the surrounding formation by mechanical breakdown of the formation. Fracturing may be carried out in wells which are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing, or other stimulation procedures, are production wells completed in oil and/or gas containing formations. However, injection wells used in secondary or tertiary recovery operations, for example, for the injection of water or gas, may also be fractured in order to facilitate the injection of fluids into such subterranean formations.
Hydraulic fracturing is accomplished by injecting a hydraulic fracturing fluid into the well and imposing sufficient pressure on the fracturing fluid to cause the formation to break down with the attendant production of one or more fractures. Usually a gel, an emulsion or a foam, having a proppant such as sand or other particulate material suspended therein is introduced into the fracture. The proppant is deposited in the fracture and functions to hold the fracture open after the pressure is released and the fracturing fluid is withdrawn back into the well. The fracturing fluid has a sufficiently high viscosity to penetrate into the formation to realize fracturing and to retain the proppant in suspension or at least to reduce the tendency of the proppant to settle out of the fracturing fluid. Generally, a gelation agent and/or an emulsifier is used to gel or emulsify the fracturing fluid to provide the high viscosity needed to realize the maximum benefits from the fracturing process.
After the high viscosity fracturing fluid has been pumped into the formation and the fracturing of the formation has been obtained, it is desirable to remove the fluid from the formation to allow hydrocarbon production through the new fractures. Generally, the removal of the highly viscous fracturing fluid is realized by "breaking" the gel or emulsion or, in other words, by converting the fracturing fluid into a low viscosity fluid. Breaking the gelled or emulsified fracturing fluid has commonly been obtained by adding a "breaker", that is, a viscosity-reducing agent, to the subterranean formation at the desired time. However, this technique can be unreliable and sometimes results in incomplete breaking of the fluid and/or premature breaking of the fluid before the fracturing process is complete. Premature breaking can decrease the number of fractures obtained and thus, the amount of hydrocarbon recovery. Further, it is known in the art that most fracturing fluids will break if given enough time at sufficient temperature and pressure. However, it is, of course, most desirable to return the well back to production as quickly as possible.
It has been suggested that a fracturing fluid is concentrated 5-7 times due to fluid loss during pumping and fracture closure. An unencapsulated breaker dissolves in the fluid and is lost along with the fluid during fluid loss. The dissolved breaker does not concentrate along with the filter cake concentration of the polymer and thus does not effectively break the filter cake. Therefore, permanent damage to the formation occurs.
There have been several proposed methods for the breaking of fracturing fluids which were aimed at eliminating the above problems. For example, U.S. Pat. No. 4,202,795 discloses a method to release a chemical into an aqueous fluid by combining the chemical with a solid hydratable gelling agent and a breaker for the gel formed by the gelling agent when hydrated. The mixture is formed into pills or pellets, preferably having size and range of from about 20 to about 40 mesh. (U.S. Sieve Series) From combining the pellets with an aqueous fluid into which the chemical is to be released, the gelling agent in the pellets hydrates and forms a protective gel around each of the pellets which prevents the release of the chemical into the aqueous fluid for the time period required for the protective gel to be broken by the gel breaker in the pellets. A large amount of hydratable gelling agent is required and the amount of hydratable gelling agent must be monitored closely.
U.S. Pat. No. 4,506,734 provides a viscosity-reducing chemical contained within hollow or porous, crushable and fragile beads. When a fracturing fluid containing such beads passes or leaks off into the formation or the fluid is removed by back flowing, any resulting fractures in the subterranean formation close and crush the beads. The crushing of the beads then releases the viscosity-reducing chemical into the fluid. This process is dependent upon the pressure of the formation to obtain release of the breaker and is thus, subject to varying results dependent upon the formation and its closure rate.
U.S. Pat. No. 4,741,401 discloses a method for breaking a fracturing fluid comprised of injecting into the subterranean formation a capsule comprising an enclosure member containing the breaker. The breaker is released from the capsule by pressure generated within the enclosure member due solely to the fluid penetrating into the capsule whereby the increased pressure causes the capsule to rupture, releasing the breaker. This method for release of the breaker would result in the release of the total amount of breaker contained in the capsule at one particular point in time. The patent examples disclose the use of the encapsulated breaker at temperatures ranging from room temperature, 65.degree. C. to 85.degree. C.
In another method to release a breaker, U.S. Pat. No. 4,770,796 teaches or suggests an acid fracturing fluid composition comprising a polymer, a crosslinking agent for said polymer, an aqueous acid and a breaker compound capable of coordinating with titanium or zirconium crosslinking agent. The breaker compound is encapsulated in a composition comprising a cellulosic material and a fatty acid and optionally a wax.
Further, U.S. Pat. No. 4,919,209 discloses a method for breaking a gelled oil fracturing fluid for treating a subterranean formation which comprises injecting into the formation a breaker capsule comprising an enclosure member enveloping a breaker. The enclosure member is capable of dissolving or eroding off upon sufficient exposure to the fluid, thereby releasing the breaker.
The typical breakers that are used in fracturing fluids are enzymes and oxidizers. Unfortunately, at temperatures below 150.degree. F., persulfates, the typical oxidizing breakers, are not effective unless used in large concentrations or used with catalysts.
Several enzymes are known to break the structure of polysaccharides or their derivatives. A great advantage of enzymes is cleanliness and effectiveness at lower temperatures. A small quantity of enzyme is capable of breaking gel quickly at mild conditions while a much larger concentration of conventional oxidizer would be required to achieve equal results. Conventional enzymes can be applied only when the fluid temperature will not exceed 150.degree. F. and when the fluid pH is in the range of 3.5 to 11. Exposure of these enzymes to higher temperatures or higher pH quickly deactivates them and usually results in a permanent enzyme degradation.
There remains a need for a method for the controlled breaking of fracturing fluids which is economical and provides controlled release of the breaker, reduces damage to the formation and facilitates well clean-up. In addition, there remains a need for a controlled release enzyme breaker which may be used in environments which would ordinarily quickly degrade the enzyme.